This invention relates to a locking support system for a self-elevating platform or jackup rig, which works in conjunction with the platform elevating system and is intended to support the weight of the platform and the storm reactions from the jackup rig legs. More specifically, this invention relates to a locking apparatus including one or more locking bars at each leg or leg chord, housed in a support foundation and secured by a retaining device, which engages the rack teeth on the leg or leg chords.
Offshore platforms have been used extensively by the oil and gas industry in continental shelf regions for oil and gas drilling, production operations, pipeline pumping stations, accommodation and service operations. Fixed offshore platforms, intended to remain in one location, are traditionally built onshore, transported by barge to the offshore location, launched and rotated into an upright position, and permanently affixed to the seafloor.
Mobile offshore vessels have been developed to meet the offshore industries needs for a facility from which to conduct drilling, production, or workover operations which will predominantly remain at any one location for a relatively short period of time, but which could also be used for extended periods of time as a production unit, accommodations unit, etc. Several different design types have been developed to meet the needs of the offshore industry including semi-submersible or floating drillships for deep water, posted barges for inland waters or bayous, and jackup platforms for shallow to moderate water depths. The jackup rig incorporates a barge shaped hull which may be towed as a floating vessel from one location to another with the majority of or all of its legs raised up through the hull above the water. Upon reaching the intended location, the elevating system will lower the legs through wells in the barge hull until firmly engaged with the ocean floor. Continued jacking down on the legs will result in penetration of the legs into the seafloor until a firm foundation for the footings is achieved such that the barge hull can then be lifted up and out of the water. Further jacking will serve to lift the hull up until the bottom of the barge reaches an elevation above the sea greater than the highest anticipated wave height during a severe ocean storm.
Henceforth, "jackup" shall pertain to any self-elevating offshore platform with one or more legs, each leg consisting of one or more chords, used for drilling, production, workover, or other offshore operations or work, which has the ability of being supported on jackable legs to the seafloor, with the capability of relocating from one offshore location to another by lowering to an afloat position, being moved to a new offshore position, and raising itself again to an elevated position. The subject invention pertains to a locking system to support the hull or legs of a jackup rig such as previously described or to support or lock in position any slidable or skidable equipment which commonly used rack and pinion systems for translation or elevation and/or any equipment which may benefit from the use of the subject invention as a locking or position holding system.
Various designs have been utilized to jack the supporting legs with respect to the platform hull. The most popular design has been the use of a rack and pinion system with one or more racks extending longitudinally along the length of each jackup unit leg or leg chords. Racks mounted to the legs mesh with pinion gears which are driven by hydraulic or electric gear drive assemblies, all mounted to frames, the combination of which is referred to as jacking units or jacking frames which are in turn mounted to the deck or internal structure of the platform hull. The pinion gears may be arranged such that the face of the pinion teeth face the center of a trussed leg with multiple chords or they may be oriented as opposed pinions with a rack mounted on each side of a leg or leg chord to engage the opposing pinions. The pinions may be, and are normally stacked vertically for all configurations to provide enough pinions to lift the desired loads. The present invention may be used with any type of jackup leg, jacking system or leg chord configuration.
The jackup rig is subjected to large environmental loadings from storms consisting of wind forces on the platform and the legs above the water in addition to ocean current and wave forces on the submerged portion of the legs. These forces result in a large overturning moment imposed on the jackup rig. The combination of these forces together with the weight of the platform can result in large interaction forces between the platform and the legs which must then be resolved at the leg to hull interface or connection.
As the design of the jackup unit evolved, three distinct design methodologies have emerged as solutions to resolution of the high interaction forces at the leg to hull interface. The three different methodologies are outlined below.
1. A "floatingjack" elevating system consisting of jacking units for each leg chord mounted on resilient pads. With this type of jack support, the moment transfer between the platform and the legs is taken primarily through horizontal forces at the upper end lower leg guides rather than vertical forces at the jack units. This method also requires substantial upper and lower guides structures to carry the large horizontal reactions back into the hull. PA1 2. A "fixed jack" elevating system consisting of jacking units rigidly affixed to the hull. In this case, the pinions will tend to absorb the majority of the overturning moment as vertical couple reactions at the pinions. The capability of the rig to resist the severe environmental conditions will be limited by the storm holding capability of the jacks, or additional pinions will have to be provided in excess of the number required for elevating the platform out of the water. This is an undesirable option given the cost of the additional pinions. PA1 3. A "locking system" consisting of a section of rack with a profile matching the leg rack, normally referred to as a "rack chock", and a multitude of mechanisms for aligning vertically and engaging horizontally the chock elements. This rack chock is commonly designed to resolve the overturning moment imposed on the jackup unit into vertical couple reactions from the leg chords and transfer it directly into the hull. The rack chocks are capable of being adjusted vertically with various mechanisms until aligned with the rack at which point the chock is engaged horizontally by same or additional mechanisms to make rigid contact with the leg. This system is normally independent of the jacking system, although they may share a common foundation, such that the load supported by the jacking pinions may be released or even be removed if required. As the locking systems and the supporting structure are commonly designed to be very stiff, the overturning moment imposed on the jackup unit by the environmental conditions is resolved as vertical load components carried by the rack chocks. The advantages of this design are a reduction in the weight of the leg, elevating pinions are sized only for the required lifting capacity and a reduction in size of the structure required to support the upper and lower guides.
Each of the three design solutions has merit and have proven to be appropriate for different operator requirements and offshore environments. However, the use of a locking or locking system of some description, for locking the jackup leg to the barge hull, has proven to be the most efficient design for the large jackup units intended for harsh environmental conditions and deep water. This has prompted significant design activity, resulting in the following United States patents: U.S. Pat. Nos. 4,255,069; 4,389,140; 4,431,343; 4,538,938; 4,589,799; 4,662,787; and Re. 32,589.
Existing locking systems to date have some structural disadvantages and difficulties in operation and maintenance. As the rack sections for the leg chords are normally manufactured using flame cutting, rack profiles are cut with some degree of imprecision or out of tolerance, resulting in dimensional variations in tooth profiles and tooth spacing. Because of these dimensional variations and the fact that the rack chock cannot be custom fitted to the leg rack due to the relative position of the barge hull being a variable over the length of the leg rack, it cannot be assured that more than one rack tooth on the leg or leg chord has made load bearing contact with the teeth of the rack chock element.
When a jackup unit is outfitted with a conventional locking system and is subjected to storm conditions, the vertical couple reactions from resolution of the overturning moment can tend to concentrate on the lower support face of the rack chock even if all teeth are in perfect contact. This imbalance may overload and damage the lower rack teeth on the chock element and the corresponding rack teeth on the leg chord. It is also difficult to vertically position the rack chock element accurately to the leg rack when using a screw jack system attached to the top, bottom and back of the chock elements or using screw jacks on wedge shaped chock elements. This can result in unbalanced load sharing on the teeth of both the rack and the chock. Some arrangements of the locking systems use screw jack elements to directly or vectorial support the reactions absorbed by the chock element from the leg or leg chord. This design can lead to damage of the screw spindles or make their operation difficult.
The design and operation of the actuating mechanisms for alignment and engagement of the rack chock element can lead to jamming and chock element removal problems. As all the existing designs depend on the locking system, independent of the jacking system, to restrain the leg chord against vertical movement in both directions, the possibility exists to jam up the actuating mechanisms by overdriving the screw jacks during final engagement.
To install the rack chocks, the normal procedure is to first vertically position the chock element, then drive the screw elements mounted on the top and bottom of the chock element against each other, thereby removing any slack in the connection of the screws spindles to the chock element. After this, the load may be transferred from the elevating pinions to the rack chocks by backdriving the pinions. For removal of the chock element, the practice is to "unload" the rack chocks by engaging the elevating pinions to slightly lift the barge hull, then use the screw mechanisms to retract the chock.
Due to tight tolerances on the screw jacks, it may prove difficult to identify when the load has been transferred from the locking system to the elevating pinions, or if the pinions have driven the chock element against the lower screw jack. As the screw jacks cannot be released until the load has been removed by the elevating pinions, removal of all the rack chock element can become a tedious and time consuming process and may have to be performed one leg chord at a time. One purpose of this invention is to simplify this process.